The present invention relates to a process for producing thermoplastic resin granules to be supplied to a granulating machine or secondary molding machine.
In the production process of a thermoplastic resin, a powdery raw material of the thermoplastic resin, which is obtained by polymerizing a plurality of materials and then drying the obtained polymer, is cooled, is then mixed with an auxiliary material such as a plasticizer or another resin material, and is then granulated by a screw extruder into pellets having grain diameters of approximately 10 mm or less. The thus obtained pellets are supplied to a secondary molding machine such as an injection molding machine or blow molding machine so as to be molded into the shape of an end product.
The powdery raw material of a thermoplastic resin just after drying and cooling as described above has a low bulk density, and thus requires unduly large pieces of equipment for its transportation and storage. Moreover, in a screw extruder, the low bulk density of the powdery raw material incurs not only large slippage between the screw and the casing thereof but also a large compression ratio, which leads to unduly high power consumption by the screw extruder when it is driven.
These inconveniences can be alleviated by first compression-molding the powdery raw material into flakes by passing it between two rolls arranged parallel with a minute gap therebetween, and then crushing the obtained flakes into granules having grain diameters of 10 mm or less.
However, the thus obtained granules do not have a satisfactorily high bulk density, and thus they do not offer desired mechanical strength.
Moreover, attempting to produce granules having uniform grain sizes within a narrow range results in low yields.
A first object of the present invention is to provide thermoplastic resin granules having a satisfactorily high bulk density.
A second object of the present invention is to efficiently provide thermoplastic resin granules having satisfactorily uniform grain sizes.
To achieve the above objects, according to a first aspect of the present invention, a process for producing thermoplastic resin granules includes compression-molding a powdery raw material of a thermoplastic resin, which is obtained by polymerizing a plurality of materials and then drying the obtained polymer, by passing the powdery raw material between two rolls arranged parallel with a minute gap therebetween at a temperature of 40xc2x0 C. or higher, and crushing the obtained compression-molded product into granules having grain diameters of 10 mm or less.
According to a second aspect of the present invention, a process for producing thermoplastic resin granules includes mixing a powdery raw material of a thermoplastic resin, which is obtained by polymerizing a plurality of materials and then drying the obtained polymer, with an auxiliary material at a temperature of 40xc2x0 C. or higher, compression-molding the powdery raw material by passing the powdery raw material between two rolls arranged parallel with a minute gap therebetween, and crushing the obtained compression-molded product into granules having grain diameters of 10 mm or less.
According to a third aspect of the present invention, in the process for producing thermoplastic resin granules of the first or second aspect of the present invention, the rolls each have a large number of identically shaped and identically sized concavities formed all over the outer circumferential surface thereof. Here, the concavities each have an elliptical opening of which the major-axis diameter measures 10 mm or less and is aligned with the direction of the circumference of the outer circumferential surface of the roll. Moreover, the concavities each have a curved surface of which the section on a plane parallel to the direction of the circumference is arc-shaped. Furthermore, the concavities are arranged in such a way that the minimum distance between any two adjacent concavities is 0.5 mm or less and that a concavity in one roll faces a concavity in the other roll across the minimum gap between the two rolls.
According to a fourth aspect of the present invention, in the process for producing thermoplastic resin granules of the third aspect of the present invention, the minimum gap between the two rolls is 0.5 mm or more and 1.0 mm or less.